This invention relates generally to an apparatus and a method for providing vapor-phase epitaxial growth on a substrate, and in particular to an apparatus and a method for providing vapor-phase epitaxial growth on a substrate using Metal Organic Chemical Vapor Deposition (MOCVD).
Thin films of metal are frequently fabricated upon solid substrates for use in the electronics and the opto-electronics industries by what is termed a Metal Organic Chemical Vapor Deposition (MOCVD) process. The process typically comprises introducing the vapor of at least one metal-organic compound, i.e. the precursor of the metal, into a reaction chamber under conditions of temperature and pressure such that the precursor decomposes to give a deposit of the metal on a solid substrate contained within the reaction chamber.
In the deposition of what is termed Group III - V semiconducting materials, vapors comprising at least one element from Group III of the Periodic Table are mixed with vapors of at least one element from Group V of the Periodic Table, and the resulting thin film material is a Group III - V semiconductor crystal. Examples of such semiconductor crystals are InP, GaAs, Al.sub.x Ga.sub.1-x As, GaAs.sub.x P.sub.1-x, In.sub.x Ga.sub.1-x As.sub.y P.sub.1-x. Group III - V semiconductor crystals are extremely useful as materials for diodes, ultra-high speed semiconductor devices, light-emitting devices, etc., and consequently the demand for these compound semiconductor crystals has increased.
In one method for producing Group III-V compound semiconductors using vapor-phase growth, an alkyl compound of a Group III element in the vapor phase and a hydride or alkyl compound of a Group V element in the vapor phase are heat decomposed in a chamber having a substrate therein. The heating causes the vapors to deposit and form Group III-V crystal on the substrate. This method is known as Metal Organic Chemical Vapor Deposition. In the production of compound semiconductors such as GaAs or Al.sub.x Ga.sub.1-x As through the MOCVD process, arsine AsH.sub.3 is widely used as the source of arsenic. By combining and decomposing arsine with an alkyl compound of a Group III element such as Gallium, GaAs crystals exhibiting satisfactory light emission characteristics can be grown.
Group III-V semiconductor compounds may also be grown using phosphorus as the Group V element and indium as the element from Group III, forming a semiconductor compound of InP. Phosphorus based compounds are widely used in many important opto-electronic devices such as lasers, light emitting diodes (LEDs), space solar cells, and detectors. Typically, phosphine, is heat decomposed in a chamber to generate the phosphorus source.
While arsine and phosphine has proven useful in producing Group III-V semiconductor compounds, both of these chemicals are toxic and classified as hazardous materials by the Environmental Protection Agency. In addition to being toxic, phosphine is also pyrophoric. Consequently, strict federal and state regulations exist on the use, storage, and disposal of arsine and phosphine. The disposal of waste byproducts of a Group III - V MOCVD process is an important environmental consideration and is highly regulated. Currently, costs to dispose of phosphorus-containing hazardous waste generated by MOCVD growth of InP or GaInP crystals are nearly equal to the cost of purchasing high purity phosphine, approximately three hundred dollars per pound. In attempting to reduce these waste byproducts, the Waste Minimization Act requires industry to implement detailed waste minimization plans for toxic materials, and imposes strict fines for non compliance.
The use of arsine and phosphine in the MOCVD process is of particular concern since, in addition to being toxic, they are also used in very high ratios relative to the volume of Group III material in the MOCVD process. For example in the production of InP in a conventional reaction chamber operating at near optimum conditions, the ratio of phosphine to the indium material is in the range of 20:1 to 60:1 at atmospheric pressure (760 torr). Many MOCVD processes operate below atmospheric pressure to improve the purity and uniformity of the semiconductor crystals. However, a reduction in chamber pressure requires an increase in the ratio of Group V material to Group III material. For instance, at a pressure of 76 torr, the ratio of the arsine or phosphine, to the Group III material, increases to the range of 200:1 to 500:1. The move to lower pressures for the MOCVD process clearly exasperates the problem, since lower pressure usually means that higher flows of the vapor must be maintained. Lower Group V to Group III ratios are possible by reducing the total main flow of the Group V gas into the chamber and thus the velocity of the gas. However, the thickness uniformity of the deposited layer may be compromised. Severe recirculation may occur within the chamber, leaving deposits inside the gas injection manifold and degrading interface abruptness.
Gallium Nitride, GaN, is one of the most promising materials for opto-electronic devices operating in the ultra-violet to ultra-blue wavelength region. Blue (LEDs) complete the primary color spectrum, and create the possibility of large full-color LED displays. While the use of these devices shows good potential, fabrication of GaN crystals with conventional MOCVD techniques requires an abundance of ammonia as the source of nitrogen. Using a conventional vertical flow reactor at low pressure and using ratios of 3000:1 for the Group V-III materials, good quality could not be grown even at temperatures of 900.degree. C. Furthermore, insufficient nitrogen utilization leads to vacancies which dominate electronic properties, material desorption which limits growth rate, and reduced growth temperature, leading to poor crystalline quality. An excess of nitrogen waste results as a byproduct of the process.
Strict environmental regulations on the use and disposal of hazardous toxins used in the production of Group III-V semiconductor compounds increase the cost of production and expose semiconductor producers to increased liability. In addressing this problem, process changes are required to reduce waste generation. In one known approach, the waste is treated after processing to convert it into non-hazardous material. This approach requires extensive waste management facilities, and, in many cases, is not practical unless the organization is certified to treat hazardous waste.
A more desirable approach is to make fundamental process changes to the MOCVD process of creating Group III-V semiconductor crystals by increasing source decomposition to reduce the amount of the toxic reactants such as arsine and phosphine used in the process, and thus also reduce proportionately the amount of the waste byproducts generated.
Accordingly, it is an object of the invention to obviate the above noted disadvantages of prior MOCVD reaction chambers and methods for utilizing the same.
A further object of the invention includes reducing the quantities of toxic reactants in a MOCVD process for fabricating semiconductor crystals.
Another object of the invention is to reduce the waste exhaust of reactant gases in a MOCVD process for fabricating semiconductor crystals.
Other objects include reducing the ratio of the amount of Group V material to Group III material, over conventional ratios, in a MOCVD process for creating high quality semiconductor devices, or the ratio of the amount of Group VI material to Group II material, over conventional ratios, in a MOCVD process for creating high quality semiconductor devices.